The invention relates to the field of carrier signal recovery, and in particular to suppression of a perturbing quadrature component of a carrier signal of an amplitude-modulated signal during the recovery of the carrier signal in a synchronous demodulator, which outputs a quadrature signal.
To receive an amplitude-modulated signal (e.g., an analog television signal), one must first select a particular channel, which is mixed by a tuner onto an intermediate frequency fZ1 (e.g., fZ1=38.9 MHz). In order to demodulate this signal by digital signal processing, one would need an analog-to-digital converter (ADC) with a very high sampling rate fAT1 and resolution b1 (e.g., fAT1=100 MHz/b1=10 bit). Therefore, the signal is mixed onto a second, lower intermediate frequency fZ2. This intermediate frequency fZ2 is ideally a frequency that corresponds to the channel raster in the HF band (e.g., fZ2=7 MHz).
Given suitable frequency selection, the intermediate signal can now be digitally converted with a much lower sampling frequency fAT2. The digitized signal is mixed by a synchronous demodulator into the base band, while the carrier frequency fT (preferably the picture carrier frequency fBT) is generated by a fully digital phase-locked loop (PLL). By further digital filtering of the resulting in-phase and quadrature component, the signal (e.g., a picture and/or sound information) is extracted.
FIG. 6 illustrates, as an example, a prior art basic layout of a digital TV receiver. The digital TV receiver 1 includes a mixer 2 that receives an intermediate frequency (IF) input on a line 3. A local oscillator 5 provides a signal on a line 4 to a second input of the mixer 2, which provides a mixed output on a line 6 to a bandpass filter 7. The bandpass filter 7 provides a filtered signal on a line 8 to an analog-to-digital converter (ADC) 9, which provides a digitized signal on a line 10 to a digital signal processor (DSP) 11.
The digitized signal on the line 10 is input to an automatic gain control (AGC) device 21 for a tuner (channel selection device), and to a synchronous demodulator 15. The synchronous demodulator 15 provides an in-phase signal output A15,I and a quadrature signal output A15,Q on lines 16, 17 respectively. Both signals are input to a filter device 18. The filter device 18 provides an in-phase signal output A18,I and a quadrature signal output A18,Q on lines 19, 20, respectively. The in-phase signal on the line 19 is input to an automatic gain control unit for video signals 23.
The automatic gain control unit for video signals 23 provides a first output designated A23,1 on a line 102 and a second output designated A23,2 on a line 28.
The first output A23,1 on the line 102 is input to the automatic gain control unit for tuner signals 21, which provides its single output A21 on line 27 to the aforementioned first output A11,1 of the digital signal processor 11.
The second output A23,2 on the line 28 is provided to the aforementioned second output A11,2 of the digital signal processor. The quadrature signal output A18,Q on the line 20 is input to an automatic gain control unit 25 for audio signals, which provides an output A25 on a line 29 to the third output A11,3 of the digital signal processor 11.
The outputs A11,1, A11,2, A11,3 on lines 27-29 respectively, are input to digital-to-analog converters (DAC) 30, 31, 32, respectively. The DAC 30 provides an analog tuner AGC signal; the DAC 31 provides an analog color video blanking signal (CVBS); and the DAC 32 provides a sound IF signal (SIF). The digital TV receiver 1 of FIG. 6 operates as follows.
The digital TV receiver 1 receives an input signal on the line 3, and the mixer 2 converts the signal into a second intermediate frequency, the so-called second IF. This second IF signal on the line 6 is bandpass filtered, to remove unwanted mixing products from the signal so it can be digitized without signal aliasing by the ADC 9.
In the digital section 11, the digitized signal on the line 10 is mixed by the synchronous demodulator 15 into the base band. The outputs A15,I, A15,Q of the synchronous demodulator 15 are presented as demodulated in-phase and quadrature signals I, Q. By further filtering and various other algorithms in the filter device 18, the video signal and the sound IF signal are obtained from the I/Q data. The tuner AGC 21 adjusts the tuner output level so that the ADC 9 connected to the input E11 of the DSPS 11 is not overmodulated. The VAGC 23 and AAGC 25 (video AGC and audio AGC) connected to the output signals on the line 19, 20, respectively, are optimally modulated for the DACs 31, 32.
If one considers the ideal case of a second IF signal supplied to the input E9 of the ADC 9 with cosine picture carrier signal with picture carrier frequency fBT, cosine picture information signal with picture information frequency fpicture and cosine sound carrier signal with sound carrier frequency fTT and cosine sound information signal with sound information frequency fsound, this can be described by the following equation:
                              u          ⁡                      (            t            )                          =                                                                              u                  ^                                BT                            ·                              cos                ⁡                                  (                                      2                    ⁢                    π                    ⁢                                                                                  ⁢                                          f                      BT                                        ⁢                    t                                    )                                            ·                              (                                  1                  +                                      m                    ·                                          cos                      ⁡                                              (                                                  2                          ⁢                          π                          ⁢                                                                                                          ⁢                                                      f                            picture                                                    ⁢                          t                                                )                                                                                            )                                                    ︸                              Picture                ⁡                                  (                                      AM                    -                    Modulation                                    )                                                              +                                                                      u                  ^                                TT                            ·                              cos                ⁡                                  (                                                            2                      ⁢                      π                      ⁢                                                                                          ⁢                                              f                        TT                                            ⁢                      t                                        +                                                                  Δφ                        TT                                            ·                                              cos                        ⁡                                                  (                                                      2                            ⁢                            π                            ⁢                                                                                                                  ⁢                                                          f                              Sound                                                        ⁢                            t                                                    )                                                                                                      )                                                                    ︸                              Sound                ⁡                                  (                                      FM                    -                    Modulation                                    )                                                                                        (                  Eq          .                                          ⁢          1                )            with:    ûBT picture carrier amplitude    m modulation index    ûTT sound carrier amplitude    ΔφTT phase deviation of the FM modulation
The sound carrier is irrelevant to the processing of the picture carrier and is therefore filtered out within the carrier recovery process. After the filtering, one gets a signal that can be described by the following equation:
                              u          ⁡                      (            t            )                          =                                                            u                ^                            BT                        ·                          cos              ⁡                              (                                  2                  ⁢                  π                  ⁢                                                                          ⁢                                      f                    BT                                    ⁢                  t                                )                                      ·                          (                              1                +                                  m                  ·                                      cos                    ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          picture                                                ⁢                        t                                            )                                                                                  )                                            ︸                          Picture              ⁡                              (                                  AM                  -                  Modulation                                )                                                                        (                  Eq          .                                          ⁢          2                )            
FIG. 7A illustrates a phasor diagram of this amplitude-modulated signal. In this case, the carrier recovery is locked onto the sum of the picture carrier 34 and the two side bands, that is the upper side band (USB), and the lower side band (LSB) representing the picture information 33.
For example, lack of symmetry in the transmission modulator or in the analog signal processing of the TV receiver 1 can perturb the picture carrier 34 with an orthogonal component 35 (FIG. 7B). Thus, from (Eq. 2):
                              u          ⁡                      (            t            )                          =                                            (                                                                                          u                      ^                                        BT                                    ·                                      cos                    ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          BT                                                ⁢                        t                                            )                                                                      +                                                                                                    u                        ^                                            q                                        ⁡                                          (                      t                      )                                                        ·                                      sin                    ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          BT                                                ⁢                        t                                            )                                                                                  )                        ·                          (                              1                +                                  m                  ·                                      cos                    ⁡                                          (                                              2                        ⁢                        π                        ⁢                                                                                                  ⁢                                                  f                          picture                                                ⁢                        t                                            )                                                                                  )                                            ︸                          Picture              ⁡                              (                                  AM                  -                  Modulation                                )                                                                        (                  Eq          .                                          ⁢          3                )            with:                ûq(t) amplitude of the perturbing component        
In the corresponding phasor representation, shown in FIG. 7B, this shows up as a phase modulation Δφ in amplitude changes of the information 33. When recovering the carrier, this phase modulation Δφ must be equalized as precisely as possible.
Demodulators according to the prior art are dimensioned such that the phase modulation is constantly adjusted by the carrier processing. This has the result, first, that rapid following of the picture carrier will correctly reconstruct the information even when an orthogonal carrier component exists, but because (e.g., in analog television) the sound carrier is mixed by the reconstructed picture carrier onto the sound intermediate frequency, it is subject to the same frequency changes, which translates into an additional frequency modulation. Secondly, the large required bandwidth of the carrier control process also adjusts the noise to maximum amplitude, which results in reducing the signal-to-noise ratio (SNR).
The digital carrier processing in the digital signal processor 11 according to FIG. 6, as is implemented in the prior art, is represented as a block diagram in FIG. 8.
In particular, one notices in FIG. 8 the synchronous demodulator 15 from FIG. 6. The synchronous demodulator 15 includes an I/Q demodulator 15a, which converts an input signal into an in-phase signal I and a quadrature signal Q. Furthermore, a circuit arrangement is provided, which obtains the picture carrier signal BT from the in-phase signal I and the quadrature signal Q and takes it directly to a first mixer 40 of the I/Q demodulator 15a and with a 90° phase shift to a second mixer 41 of the I/Q demodulator 15a. 
Specifically, the I/Q demodulator 15a is constructed in the usual manner. It comprises one input E15 and two outputs, namely, an in-phase signal output A15,I and a quadrature signal output A15,Q. The input E15 is provided on the line 14 to the first mixer 40, and the second mixer 41. Both mixers 40, 41 have an additional second input E40,2, E41,2, which receive the picture carrier signals BT in the above-mentioned manner.
The mixer 40 provides an output on a line 45 to a first low-pass filter 38. The mixer 41 provides an output on a line 46 to a second low-pass filter 39.
The first low-pass 38 provides the in-phase signal output A15,I on the line 16, and the second low-pass 39 provides the quadrature signal output A15,Q on the line 17.
The signals on the lines 16, 17 are input to a low-pass filter 51, which filters each of the signals to provide outputs A51,1 and A51,2 on lines 52, 53, respectively to a computer unit 54 corresponding to the inputs E51,1 and E51,2, respectively.
The computer unit (i.e., CORDIC) 54 provides outputs A54,1, A54,2 on lines 55, 56 to an automatic controller 37.
One or more outputs A37 of the computer unit 37 are provided on lines 57 to a digital I/Q oscillator 36, which provides signals on the lines 58 to the mixers 40 and 41.
The low-pass filter 51 selects the picture carrier, which lies here in the baseband (fBT≈0 Hz). The subsequent Coordinate Rotation Digital Computer (CORDIC) 54 determines, from the low-pass filtered I/Q pairs of values (i.e., the signals on the lines 52, 53), the phase 55 and the amplitude 56. The phase on the line 55 constitutes the phase difference between the picture carrier of the received signal on the line 14 and the local carrier on the line 58 of the I/Q oscillator 36. The phase on the line 55 is converted in the automatic controller 37 to a correction signal on the line 57, in order to follow the local I/Q oscillator 36. After several iterations (loop passes), the carrier on the line 58 is matched to the received carrier on the line 14.
In the above-described digital implementation of the carrier control process, a bandwidth comparable to the analog solution can only be achieved with difficulty. The bandwidth is limited by the relatively large signal delay within the automatic control loop (filtering, phase and amplitude measuring with Cordic algorithm).
Upon receiving an analog television signal, this has effects on the demodulated video signal as well as the demodulated sound carrier, since the latter is also frequency-modulated by the adjustment of the PLL. FIG. 9 illustrates the impact of a quadrature component in the in-phase signal on the demodulated video signal.
The sound carrier is converted by the local I/Q oscillator 36 into the sound intermediate frequency. If, due to an orthogonal perturbing component of the picture carrier, a phase modulation of the picture carrier occurs, this translates into a frequency modulation of the sound intermediate frequency carrier, since the local carrier 58 follows the phase modulation.
FIGS. 9A-9B illustrate part of a CVBS signal. In FIG. 9A, the horizontal synchronization pulse is clearly recognizable (at around 650<t<750). FIG. 9B shows the same case with an additional orthogonal perturbing component. The PLL loop in this case is not fast enough to demodulate the signal in an error-free manner. The horizontal synchronization pulse is distorted so much that a connected TV set cannot generate any stable picture from it.
Therefore, there is a need for a method and a circuit arrangement in which unperturbed demodulation of the signal is improved during the processing of the carrier, even when the transmitters are poorly aligned. There is also a need to estimate and compensate for the orthogonal perturbation.